1. Field of the Invention
The present invention relates to a deposited film forming method and a deposited film forming apparatus for continuously forming a deposited film on an elongated substrate by a plasma CVD method.
2. Related Background Art
As a deposited film forming apparatus for continuously forming a deposited film on a surface of a belt-like substrate, there is conventionally known a deposited film forming apparatus using a continuous plasma CVD method which adopts, for example, a roll-to-roll system disclosed by U.S. Pat. No. 4,400,409.
This apparatus is conceived to be capable of continuously forming an element which has a large area and a semiconductor junction by disposing a plurality of glow discharge regions, arranging the glow discharge regions along a path through which a sufficiently long belt-like substrate having a desired width passes, and continuously conveying the above-described substrate in a longitudinal direction while depositing and forming a semiconductor layer with a required conductivity in the glow discharge regions. The roll-to-roll system can therefore be said as a method which is suited for mass production of semiconductor elements having large areas.
In the conventional CVD method, radio frequency (RF) discharge is widely used as glow discharge plasma exciting means for forming a deposited film by decomposing a raw material gas. On the other hand, attentions have recently been paid to a plasma process which uses a microwave.
The microwave which has a frequency higher than that of RF used in the conventional case is capable of enhancing an energy density and is suited for efficient generation and maintenance of plasma.
For example, U.S. Pat. Nos. 4,517,223 and 4,504,518 disclose methods for depositing and forming a thin film on a substrate having a small area in microwave glow discharge plasma at a low pressure, and it is conceived that the microwave is capable of causing discharge at a lower pressure in comparison with RF and prevents polymerization of active species which constitute a cause for degradation of film characteristics, thereby making it possible not only to obtain a deposited film of high quality but also to suppress production of powders such as polysilane in plasma and remarkably enhance a deposition rate.
For mass production of semiconductor junction elements, for example, solar cells, it is necessary and indispensable to enlarge the deposition region by the deposited film forming apparatus by using the continuous plasma CVD method employing the roll-to-roll system.
Deposited films manufactured by apparatuses using the conventional microwave CVD method and RF plasma CVD method have larger ununiformity in a width direction as the deposition rate and the discharge region become larger.
Furthermore, the deposition rate is remarkably changed dependently on a kind of a discharge energy for generating plasma, discharge conditions (for example, discharging electric power and discharging frequency), a flow rate, a flow velocity, a pressure of a raw material gas in a vacuum container and the like, but changes in raw material gas concentration distribution and plasma density distribution are not taken into consideration, thereby posing a problem that a deposited film which has a uniform thickness and uniform quality cannot be formed in a large deposition region.
For example, in the case of pursuing a deposited film having characteristics for a solar cell by the microwave plasma CVD method, it is necessary to lower microwave electric power and enhance an RF bias introduction electric power. However, a deposited film obtained in such conditions has a thickness distribution which is changed depending on a distance from a microwave introduction position, that is, the deposited film is thinned as its portion is farther from the microwave introduction position.
When the plasma CVD method is applied to a deposited film having a large area, it is difficult to obtain a deposited film which has an uniform thickness and an uniform quality over a large area.
As describe above, the apparatus for continuously forming a semiconductor deposited film by the conventional roll-to-roll system poses a problem that the characteristics of a photovoltaic element are liable to be ununiform in a width direction of a substrate when a deposited film is formed by the microwave plasma CVD method which is capable of forming a film at a high speed.
In view of the above-described problem, an object of the present invention is to provide a deposited film forming method and a deposited film forming apparatus which are capable of forming a photovoltaic element having no ununiform characteristics by depositing semiconductor layers without ununiform thickness and ununiform properties.
In order to accomplish the above-described object, a deposited film forming method according to the present invention is a deposited film forming method for generating plasma in a plurality of successive vacuum containers and continuously forming a deposited film on a belt-like substrate while continuously moving the belt-like substrate in its longitudinal direction, wherein an opening of a discharge container is adjusted with opening adjusting plates having shapes set so as to reduce thickness ununiformity in a width direction of the substrate on the basis of a measurement of a deposition rate distribution.
For the above-described deposited film forming method, when a distance of a discharge space in a conveying direction is represented by xn, a distance of the discharge space in a direction perpendicular to the conveying direction is represented by yn, a deposition rate at an optional point (xn, yn) in the discharge space is represented by d(xn, yn), a deposited film thickness at y=yn, XPnxe2x89xa6xnxe2x89xa6xQn in the discharge space is represented by xcex4n(x, yn), wherein n=3, 4, 5, 6, . . . , and when a substrate conveying rate is represented by v and an ideal thickness of the deposited film is designated by xcex4,
dn(x, yn)=dn(vt, yn)=ant2+bnt+cn(∵x=vt)
(Which is an approximate expression of the deposition rate)                                           δ            n                    ⁡                      (                          vt              ,                              y                n                                      )                          =                                            ∫              Pn              Qn                        ⁢                                                            d                  n                                ⁡                                  (                                      vt                    ,                                          y                      n                                                        )                                            ⁢                              ⅆ                t                                              =          δ                                    (        1        )            
Then, a point An (vPn, yn) and a point Bn (vQn, yn) which satisfy the formula (1) are obtained.
A quadratic curve passing the point An:
x=F1(y)xe2x80x83xe2x80x83(2)
A quadratic curve passing the point Bn:
x=F2(y)xe2x80x83xe2x80x83(3)
It is preferable to set both ends of the opening of the discharge container so as to have a curve satisfying the formula (2) and a curve satisfying the formula (3) and limit the deposition region of the belt-like substrate by adjusting the opening of the discharge container. In the present invention, the both ends of the opening mean two portions defining end portions of the opening in the width direction of the substrate.
Furthermore, it is preferable to set a shape of the above-described opening adjusting plate so that the both ends of the opening of the discharge container have an arc passing the above-described point An and an arc passing the above-described point Bn which are determined from a deposition rate distribution in the width direction of the belt-like substrate and limit the deposition region of the belt-like substrate by adjusting the opening of the discharge container with the opening adjusting plate.
Furthermore, it is preferable to set the shape of the above-described opening adjusting plate within xc2x110% of a shape of an opening determined from the deposition rate distribution in the width direction of the belt-like substrate.
Furthermore, it is preferable that the above-described deposited film is formed by the microwave plasma CVD method.
Furthermore, it is preferable to set the shape of the opening adjusting plate so that ununiformity of a film thickness distribution of the deposited film is within 10% in the width direction of the belt-like substrate.
Furthermore, it is preferable to adopt the roll-to-roll system in which a belt-like substrate from a roll-like wound substrate are continuously moved through a plurality of discharge containers.
On the other hand, a deposited film forming apparatus according to the present invention is a deposited film forming apparatus for generating plasma in a plurality of successive vacuum containers and continuously forming a deposited film on a belt-like substrate while continuously moving the above-described belt-like substrate in its longitudinal direction, wherein opening adjusting plates having shapes set so as to reduce ununiformity of a film thickness of the deposited film in the width direction of the substrate on the basis of a measurement of a deposition rate distribution are disposed in an opening of discharge container.
For the above-described deposited film forming apparatus according to the present invention, when a distance of a discharge space in a conveying direction is represented by xn, a distance of the discharge space in a direction perpendicular to the conveying direction is designated by yn, a deposition rate at an optional point (xn, yn) in the discharge space is denoted by d(xn, yn), a deposited film thickness at y=yn, xpnxe2x89xa6xnxe2x89xa6xQn in the discharge space is represented by xcex4n(x, yn), wherein n=3, 4, 5, 6, and when a substrate conveying rate is represented by v and an ideal deposited film thickness is designated by xcex4,
dn(x, yn)=dn(vt, yn)=ant2+bnt+cn(∵x=vt)                                           δ            n                    ⁡                      (                          vt              ,                              y                n                                      )                          =                                            ∫              Pn              Qn                        ⁢                                                            d                  n                                ⁡                                  (                                      vt                    ,                                          y                      n                                                        )                                            ⁢                              ⅆ                t                                              =          δ                                    (        1        )            
Then, a point An (vPn, yn) and a point Bn (vQn, yn) which satisfy the formula (1) are obtained.
A quadratic curve passing the point An:
x=F1(y)xe2x80x83xe2x80x83(2)
A quadratic curve passing the point Bn:
x=F2(y)xe2x80x83xe2x80x83(3)
It is preferable to set both ends of the opening of the discharge container so as to have a curve satisfying the formula (2) and a curve satisfying the formula (3), respectively, and dispose an opening adjusting plate in the opening of the discharge container to limit the deposition region of the belt-like substrate.
Furthermore, it is preferable to set a shape of the above-described opening adjusting plate so that both the ends of the opening of the discharge container have an arc passing the above-described point An and an arc passing the above-described point Bn which are determined from a deposition rate distribution in the width direction of the belt-like substrate, and dispose an opening adjusting plate in the opening of the discharge container to limit the deposition region of the belt-like substrate.
Furthermore, it is preferable to set the shape of the above-described opening adjusting plate so as to be within xc2x110% of a shape of an opening adjusting plate determined from the deposition rate distribution in the width direction of the belt-like substrate.
Furthermore, it is preferable that the above-described deposited film is formed by a microwave plasma CVD apparatus.
Furthermore, it is preferable to dispose an opening adjusting plate having such a shape as to limit ununiformity in a film thickness distribution of the above-described deposited film within 10% in the width direction of the belt-like substrate.
Furthermore, it is preferable to configure the deposited film forming apparatus of a roll to roll system type apparatus in which a belt-like substrate from a roll-like wound substrate is continuously moved through a plurality of discharge containers.
The deposited film forming method which uses the plasma CVD method generates a film thickness distribution of a deposited film when a discharge space is expanded so as to enlarge a film forming region. The distribution is largely changed depending on a microwave introducing method and conditions for forming the deposited film. A deposition rate distribution is largely different, for example, between a case where a microwave electric power is introduced in one direction and a case where the microwave electric power is introduced in two directions.
According to the present invention, a deposition rate distribution is measured and a shape of the opening adjusting plate is determined so as to make uniform a deposited film thickness in the width direction of the substrate. By disposing the opening adjusting plate in the opening of the discharge container, it is possible to shield the substrate from the discharge region and limit a deposition region on the belt-like substrate. This method makes it possible to form a deposited film which has a uniform thickness over a large area.
Even when film forming conditions are modified, in order to optimalize deposited film forming conditions, a shape of the opening adjusting plate is determined once again to be able to form a deposited film having a uniform thickness.
Furthermore, the present invention provides a film forming method for generating plasma in a plurality of successive vacuum containers and continuously forming a deposited film on a belt-like substrate while continuously moving the above-described substrate in its longitudinal direction,
wherein both ends of the opening of the discharge container are set so as to have a curve satisfying the following formula (4) and a curve satisfying the following formula (5), respectively, to adjust the opening of the discharge container, thereby limiting a deposition region of the belt-like structure, and
wherein when a distance of a discharge space in a conveying direction is represented by xn, a distance of the discharge space in a direction perpendicular to the conveying direction is represented by yn, a deposition rate at an optional point (xn, yn) in the discharge space is represented by d(xn, yn), a deposited film thickness at y=yn, xPnxe2x89xa6xnxe2x89xa6xQn in the discharge space is represented by xcex4n(x, yn), and n=3, 4, 5, 6, . . . , and when a substrate conveying rate is represented by v and an ideal thickness of the deposited film is represented by,
xe2x80x83dn(x, yn)=dn(vt, yn)=ant2+bnt+cn(∵x=vt)                                           δ            n                    ⁡                      (                          vt              ,                              y                n                                      )                          =                                            ∫              Pn              Qn                        ⁢                                                            d                  n                                ⁡                                  (                                      vt                    ,                                          y                      n                                                        )                                            ⁢                              ⅆ                t                                              =          δ                                    (        1        )            
a point An (vPn, yn) and a point Bn (vQn, yn) which satisfy the formula (1) are obtained, and
a quadratic curve passing the point An is x=F1 (y) . . . (2), and
a quadratic curve passing the point Bn is x=F2 (y) . . . (3), and
on the basis of formulas (2) and (3), the following inequality formulas are obtained, respectively:
1.1F1(y)xe2x88x920.1F2(y)xe2x89xa6xxe2x89xa60.9F1(y)+0.1F2(y)xe2x80x83xe2x80x83(4)
0.9F2(y)+0.1F1(y)xe2x89xa6xxe2x89xa61.1F2(y)xe2x88x920.1F1(y)xe2x80x83xe2x80x83(5)
Furthermore, the present invention provides a deposited film forming apparatus for generating plasma in a plurality of successive vacuum containers and continuously forming a deposited film on a belt-like substrate while continuously moving the above-described substrate in a longitudinal direction,
wherein both ends of the opening of the discharge container are set so as to have a curve satisfying the following formula (4) and a curve satisfying the following formula (5), respectively, to adjust the opening of the discharge container, thereby limiting a deposition region of the belt-like structure, and
wherein when a distance of a discharge space in a conveying direction is represented by xn, a distance of the discharge space in a direction perpendicular to the conveying direction is represented by yn, a deposition rate at an optional point (xn, yn) in the discharge space is represented by d(xn, yn), a deposited film thickness at y=yn, xPnxe2x89xa6xnxe2x89xa6xQn in the discharge space is represented by xcex4n(x, yn), and n=3, 4, 5, 6, . . . , and when a substrate conveying rate is represented by v and an ideal thickness of the deposited film is represented by,
dn(x, yn)=dn(vt, yn)=ant2+bnt+cn(∵x=vt)                                           δ            n                    ⁡                      (                          vt              ,                              y                n                                      )                          =                                            ∫              Pn              Qn                        ⁢                                                            d                  n                                ⁡                                  (                                      vt                    ,                                          y                      n                                                        )                                            ⁢                              ⅆ                t                                              =          δ                                    (        1        )            
a point An (vPn, yn) and a point Bn (vQn, yn) which satisfy the formula (1) are obtained, and
a quadratic curve passing the point An is x=F1 (y) . . . (2), and
a quadratic curve passing the point Bn is x=F2 (y) . . . (3), and
on the basis of the formulas (2) and (3), the following inequality formulas (4) and (5) are obtained, respectively:
1.1F1(y)xe2x88x920.1F2(y)xe2x89xa6xxe2x89xa60.9F1(y)+0.1F2(y)xe2x80x83xe2x80x83(4)
0.9F2(y)+0.1F1(y)xe2x89xa6xxe2x89xa61.1F2(y)xe2x88x920.1F1(y)xe2x80x83xe2x80x83(5).